1. Field of the Invention
One aspect of the present invention relates to dot enhancing compositions for negative working photographic systems. More particularly, certain embodiments of the invention relate to utilization of photographic elements containing novel dot enhancing compositions to improve dot quality in letterpress and offset lithography.
2. Discussion Of The Art
High contrast negative-working silver halide photographic elements, together with film emulsions and appropriate developers are known in the art and are particularly useful in forming half tones in letterpress and offset lithography. Rather than reproducing tones by varying the amount of ink, letterpress and offset lithography conventionally convert halftones into a pattern of small and clearly defined dots, wherein darker tones are formed by increasing dot size and lighter tones by decreasing dot size.
It is highly desirable that each dot display the highest possible optical density and that the dots be well formed with the fringe area around each dot displaying sharp contrast such that optical density drops very quickly as a function of distance from the edge of the dot. This characteristic is often referred to as "edge gradient". A dot with high density and good contrast is said to be a "hard dot".
In addition to the foregoing characteristics, it is also important that the edge of each dot be sufficiently smooth to avoid bridging with neighboring dots when lighter tones are being reproduced. This smoothness may be measured by determining the percentage of darkened surface area on a photographic element at which bridging first occurs. It is desirable for dot smoothness to substantially avoid bridging at less than 40 percent and more preferably 45 percent or as close to 50 percent as possible. Avoidance of bridging near the 50 percent level requires a smooth and well-formed dot. A hard dot which also achieves high smoothness enables high accuracy tone reproduction needed in the industry.
In the prior art, various hydrazines, particularly formyl hydrazines, have been used as developing agents in order to enhance dot quality. It is believed that these prior art compounds promote infectious development and desirably increase density and contrast. Formyl phenylhydrazines and various aryl formyl hydrazides have been utilized as part of the photographic film emulsion in hydrophilic colloid layers of negative working photographic materials, and it has occasionally been suggested that they be used as part of a developing solution. Systems using these prior art compounds have succeeded in producing dots with good density and contrast. However, these dots do not necessarily display the smoothness necessary for highly accurate tone reproduction.
Another problem with prior art systems is the undesirable occurrence of "pepper effect", which may result when silver is undesirably reduced in the absence of exposure of film to light. Hence, dark spots or "pepper" may appear at unexposed positions on the film which should not be darkened. In many prior art systems this effect may become more pronounced over time as developing solutions are broken down by contact with atmospheric oxygen. A problem with prior art systems is that high density and good contrast have been difficult to achieve while simultaneously providing smooth edges and retarding pepper effect.